1. Field of the Invention
The present invention relates generally to a heat exchanger applicable to an air conditioner or an industrial refrigeration unit and, more particularly, to a fin-tube type heat exchanger equipped with a stack of a plurality of heat transfer fins mounted on heat transfer tube groups in a heat exchanging relationship.
2. Description of the Prior Art
A fin of a conventional fin-tube type heat exchanger is provided with a multiplicity of slats generally referred to as a louver that are raised from the upper surface of fin. A performance is enhanced by making the slats contact with fresh air as much as possible. The contact of the slats with a great deal of fresh air is attained by reducing a boundary layer (strictly, temperature boundary layer) in which an air flow stays, by a decreasing widths of the slats or arraying downstream slats in proper positions such that the boundary layers formed upstream exert a less influence on the downstream slats. If the slat width is too reduced, forming of the fin becomes difficult. Utilized often is a method of improving the fin performance with a contrivance of the arrangement of slats. If the ideas of enhancing the fin performance depending upon the structure of louvers are the same, however, there can be seen considerably different ideas in configurations of the such as, for example, size, weight and air-flow resistance required concomitantly with the heat exchanger.
In order to reduce both size and weight of the fin, Japanese Patent Publication No. 59-26237 proposes a fin having, as shown in FIGS. 38 and 39, a bent configuration as viewed in a cross-section (taken in a direction of air flow). This fin has high stiffness in the lengthwise direction of the fin because the slats of the louver are bent. The fin, therefore, presents a less deformation even if the fin plate is made of a thin sheet metal. Thus, the louver can be disposed in a proper position; however, the air flow is, disturbed due to V-shaped slats of the louver. There exists a possibility of increasing the air-flow resistance in a high air velocity area. It is therefore desirable that the fin be designed for use with a low air velocity.
Namely, the bent fin can be termed as a low-deformation and high-performance fin suited to a low air velocity. On the other hand, as shown in FIGS. 40 and 41, Japanese Unexamined Patent Publication No. 61-6588 discloses a fin advantageous to a reduction in air-flow resistance, wherein plural pairs of slats having different shapes as viewed in sections taken along longitudinal direction of the louver are arranged in the direction of the air flow (the fin based on this structure is called an "accordion fin"). This fin does not disturb the air flow because the slats of the fin are planar as viewed in a section taken along the air-flow direction. The air-flow resistance is therefore low even at the high air velocity. In this fin, however, V-shaped means or the like for increasing the longitudinal stiffness are not provided in the louver. The fin therefore tends to be deformed in the longitudinal direction thereof. In such elongated fins the number of heat transfer tubes arranged in the longitudinal direction of the fin is large, the entire fin tends to be flexed to cause a possibility that the tube pitch deviates from a design value. Consequently, it is difficult to stack and convey the fins and, consequently, the accordion fin is limited to a short fin with a relatively small amount of deformation.
In the above-described fins of the prior art, the louver is deformed differently in different parts of the fins. Influences exerted on the fin performance are correspondingly different in different parts of the fins. Further, the velocity of air flowing between the adjacent fins of the fin-tube type heat exchanger is not uniform while the air flows across the widths of the fins and because the air flow results in a contraction flow and an expansion flow depending on the existence of pipes.
Then, for preventing a decline of the performance due to the deformation of the louver, there arises a problem that the slats in the portion subjected to a less deformation and having no necessity for enhancing the stiffness are bent with a resultant increase in the air-flow resistance. Reversely, when the louver is constructed of only the slats having a planar configuration, a problem is also caused that fabrication and workability of the fin are reduced due to lack of the fin stiffness.